1. Field of the Invention
The present invention relates to automatic control methods and devices for rolling mills, and particularly to automatic strip thickness control methods for obtaining a rolled product having a desired thickness by means of a Sendzimir mill for rolling an electrical steel sheet or stainless steel sheet, apparatuses for carrying out the methods, an automatic reduction rate control method for a rolling mill for rolling a material to be rolled at a predetermined rate of reduction, and an apparatus for carrying out the method.
2. Description of the Prior Art
In recent years, it has become desirable to provide improved accuracies in plate thickness in the rolling of steel sheets by means of rolling mills, particularly, in the cold rolling of thin steel sheets such as an electrical steel sheet and a stainless steel sheet by means of Sendzimir mills, and consequently, it is desired to improve the accuracy in strip thickness control. As a method of controlling the strip thickness or a device therefore, there have, heretofore been employed automatic gauge control (hereinafter referred to as "AGC") for diminishing to zero the deviation in the output side strip thickness from a predetermined desired uniform gauge thickness and automatic reduction rate control (hereinafter referred to as "ARC") for rolling a material at a constant rate of reduction. The former AGC includes various types such as the so-called BISRA AGC, Feedback AGC, Massflow AGC and Forward AGC, each of which suffers from the following problems. Firstly, when the gain of the screwdown system is raised to improve the responsibility in controlling a loop system in the devices as described above, a hunting phenomenon due to an overshoot from the desired value in the reduction cylinder occurs, so that the accuracy in strip thickness is decreased as compared with the preceding pass. Secondly, when any one of the abovedescribed AGC's are is continuously employed in a reversing mill, the accuracy in strip thickness does not necessarily improve in a pass, as compared with the result of rolling in the preceding pass, as the number of the rolling passes is increased.
To solve the first problem, a simple method has been adopted of providing a dead band, and a more advanced method of the optimum rolling, in which the direction of reduction is changed over before the reduction cylinder has the overshot. In the former method, an output signal for control is emitted only when a preset value of deviation is exceeded, e.g., by .+-.1.about..+-.2 .mu.m, however, there is such a disadvantage that, decreased dead band results in lowered effects and increased dead band does not result in the required accuracy in strip thickness, so that practically, it is difficult to attain an improvement in the accuracy in strip thickness by use of this method alone.
Particularly, in view of modern accuracy requirements in strip thickness for electrical steel sheet, difficulties are felt in providing a dead band larger than .+-.1.about..+-.2 .mu.m in the aforesaid example, so that the provision of the dead band alone does not necessarily result in satsifactory effects. The latter method, optimum reduction, in which complex calculations are performed to switch control points, is an excellent automatic strip thickness control method. However, if it is required to shorten the sampling time, there may occur such a disadvantage that the time required for processing lacks because the calculations are complex.
To solve the second problem, the material is rolled at a low rolling speed when an AGC is continuously employed during a plurality of rolling passes. However, this method is not preferable because it results in lowered productivity. Further, in the case of the reversing mill, it is estimated that unsatisfactory controlling operation results when the signal of strip thickness of the preceding rolling pass is fed to the screwdown system of a constant response speed as an input (deviation) for the succeeding rolling. A hunting phenomenon may occur due to phase delay because of high frequency components over the responsibility of the screwdown system, and may result in lowered accuracy of strip thickness. To avoid this defect, the response of the screwdown system should be varied in accordance with the number of rolling passes. However, it is not easy to do so with the reversing mill of many passes because of the upper limit of the response of the screwdown system.
In contrast to the above, the aforesaid AGC corrects the input deviation by the value corresponding to the rate of reduction, whereby the output value of rate of reduction is low as compared with the aforesaid AGC's, so that the load of the screwdown system can be low, the responsibility enhanced and the stability improved. Particularly, if the strip thickness is controlled within a certain deviation (.+-.10 .mu.m, .+-.5 .mu.m, for example) in the preceding rolling, then the deviation can be decreased in accordance with the rate of reduction, so that a comparatively moderate control can be effected. With AGC in which a zero deviation is desired, an overshoot in the screwdown system takes place around the desired value, i.e., the so-called hunting phenomenon occurs. With ARC, such a phenomenon does not take place. However, with this ARC, there is a problem that the accuracy in controlling the strip thickness is lowered in the case of a high value of deviation.
For the materials to be rolled, respective pressure control reduction rates are preset according to the applications of the materials, and the rate of reduction has a great influence on the mechanical properties and other characteristics of a product. For example, in the temper rolling of a non-oriented electrical steel sheet, the rate of reduction displays a great influence on the magnetic characteristics. Consequently, with certain types of materials, there are some cases where the rate of reduction in the direction of rolling is required to be set at a predetermined value. A specific method of ARC, in which the rate of reduction is controlled at a predetermined value for use in the case as described above, is one in which the rate of reduction is detected to control the roll gap to become equal to the desired rate of reduction in the same manner as in the ordinary strip thickness control, in which the strip thickness at the output side of the mill is measured to control the rate of reduction. As the methods of measuring the rate of reduction in this case, there have, heretofore, been known a method of measuring the rate of reduction by use of a strip thickness gauge, a method of measuring the percentage of elongation from the strip length or strip speed by use of deflector rolls, etc. With ARC by use of the former, the position, where the strip thickness gauge is provided, is apart from the positions of work rolls, whereby a delay in time takes place due to the travel of the material therebetween, thus deteriorating the controllability. With ARC by use of the latter, errors occurring due to slip between the deflector rolls and the steel sheet and the difference between the diameters of rolls hamper the accurate measurement of the percentage of elongation. Further, it is difficult to correct the errors during rolling. Moreover, no data on the input side strip thickness are rendered, whereby the controllability on the disturbance such as a change in input thickness is low, which, in an extreme case, may cause an accident of a sheet break. In addition, another ARC obtains a constant rate of reduction by rolling at a predetermined pressure by use of an accumulator and rolls of low elasticity. However, particularly, the mills having a multi-roll arrangement have hysterisis due to friction and looseness, thus presenting a disadvantage that a constant rate of reduction is not easily obtainable.